Portable electronic devices continue to get smaller and incorporate more functions, such as traditional personal digital assistant (“PDA”) functionality with cellular telephony and wireless email capability. In addition to functions oriented toward the business user, it is also known to incorporate music and video players as well as camera applications for consumer market devices.
Conventional film cameras use a photosensitive film to capture an image, whereas digital cameras use electronic photosensors such as charge coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) chips. The term “photosensor” as used in this specification means any device(s) or material(s) capable of receiving and capturing radiant energy, and being at least partially capable of converting the radiant energy into electronic signals that become a virtual representation of the optical image. A CCD or CMOS “camera-on-a-chip” includes an array of very fine electronic “picture elements” or “pixels” arranged in horizontal rows and vertical columns that define an image resolution matrix.
U.S. Pat. No. 5,841,126 describes an exemplary camera chip that may be incorporated into a portable electronic device.
Most camera applications utilize a “rolling shutter” mechanism for clocking out successive rows of pixels from the array. Typically, the rows of pixels are refreshed or reset in sequence, starting at the top of the image and proceeding row by row to the bottom. When this refresh process has progressed a predetermined distance down the image, the readout process begins, wherein data from rows of pixels are read out in sequence, starting at the top and proceeding row by row to the bottom in the same manner and at the same speed as the refresh process.
The rate at which the pixel data is clocked out depends on three factors: 1) frame size (larger resolutions require higher clock rates); 2) frame rate (higher frame rates require higher clock rates; and 3) amount of digital zoom applied (higher zoom levels require higher clock rates).
Data clocked out of the array at high data rates places significant demands on baseband processing capabilities. Also, there is a practical limit to the rate at which the baseband processor can accept data from the photosensor array. Baseband processing capabilities therefore play an important role in determining the size of the camera viewfinder. In this specification, the term “viewfinder” refers to a digital viewfinder, implemented as a software application executed by a processor, in conjunction with the photosensor array and a display. The software application mimics the function of an optical viewfinder that is used on a conventional camera.
Optimizing one of the three factors discussed above results in limitations being imposed on the other two factors. Thus, a larger viewfinder provides better aesthetics at the expense of a slower frame rate (or refresh rate) and lower zoom level. Conversely, a smaller viewfinder provides higher frame rate and/or zoom level at the expense of aesthetics.